What is a JAR file? Learn about JAR files here
JAR stands for Java Archive. It is a compressed file, which means that it is a file that has been reduced in size. This is often done to speed up distribution over the Internet and to save storage space.
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JAR file details
A JAR file is a type of compressed file, which is actually using the same compression algorithms and format as ZIP files. ZIP files are the most popular form of compressed file available.
JAR files normally contain a number of Java Classes and some metadata (a manifest file). Java classes are part of applications which are written in Java, a popular programming language. Some are able to run in your browser (Java applets), and some are standalone Java applications for which you need a Java Runtime Environment to run.
Opening JAR files
If you are trying to open or create JAR files on a Windows PC, then you will need a tool like BitZipper. After installing BitZipper, you can open a JAR file simply by double-clicking it or by right-clicking on it in Windows Explorer, or by starting BitZipper and opening the JAR file the same way you open a document in your word processor.
Notice that this is for viewing what is inside a JAR file, not for starting the Java application in the JAR file.
If you are looking for technical details and the complete history of the .JAR file format, please refer to this Wikipedia article and the JAR specifications at Sun Microsystems.